Integral plastic fastener

ABSTRACT

An integral plastic fastener for a plastic article in the form of a cylindrical lug extending from the article and a hollow core in the central portion of the lug, the length of the core being greater than the length of the lug, extending into the plastic article. The diameter of the lug preferably is the same along its length.

DESCRIPTION

1. Field of the Invention

This invention relates to integral plastic fasteners, especially forplastic covers to be attached to seat belt retractors.

2. Description of the Prior Art

Devices such as covers for rewind springs and for tension relievingmechanisms have been attached to seat belt retractors by a variety ofmeans. Such means include the use of integral plastic fasteners. Forexample, plastic push pin-type fasteners have been molded into retractorspring covers. In addition, split pin-type fasteners have beensuggested, as disclosed in U.S. Pat. No. 4,204,652 to Cislak et al., forexample. Furthermore, integral plastic fasteners for retractor springcovers have been utilized which include outwardly tapered legs.

The prior fasteners, although generally acceptable, are either difficult(and hence relatively expensive) to mold or have extraction strengthswhich are only moderately high. It would be desirable to provide anintegral fastener which was easy to mold and yet offered high extractionstrengths.

SUMMARY OF THE INVENTION

In accordance with this invention, there is provided, in a moldedplastic article adapted to be attached by an integrally molded plasticfastener to a cooperating part having an aperture for receiving thefastener, the improvement comprising:

the plastic article having a first surface, an opposite second surfaceand a solid central portion between the surfaces;

the fastener comprising an integral lug extending from the first surfaceoutwardly and terminating in a distal end, the portion of the firstsurface which intersects the lug defining a shoulder adapted to surroundthe aperture, the central portion of the lug having an axial hollowcore, the core being surrounded by a solid wall portion, the hollow coreextending from a first end adjacent the distal end to a second end whichis located interiorly of the first surface so as at least to extend intothe solid central portion of the plastic article, the length of the corebeing greater than the length of the lug; and

the outside lateral dimension of the lug being substantially the sameadjacent the first surface and adjacent the distal end.

Preferably, the cooperating part is a seat belt retractor and the moldedplastic article is a cover for a part of the retractor. Also, preferablythe inner walls of the core taper slightly inwardly from the distal endto the second end, and the second end terminates within the centralportion of the plastic article. In addition, it is preferred that thecore has a generally circular shape as does the outer wall of the lug.

The integral fastener of this invention is easy to mold and exhibitsvery good extraction strength, even after exposure to heat and humidity.In addition, the insertion force is reduced over prior art fasteners andthe extraction strength is retained over a wide range of interferencefits. Thus, the fastener does not require high tolerances. The lug isretained within the aperture solely by an interference fit due tocompression of the plastic material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross-section view of the integral plasticfastener of this invention.

FIG. 2 is an end view of the fastener of FIG. 1 taken along line 2--2.

FIG. 3 is a perspective view of a seat belt retractor frame and springcover mounted thereon by the fastener of this invention.

FIG. 4 is an enlarged cross-sectional view of a section of FIG. 3showing the fastener in place.

FIG. 5 is a view of a spring cover of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, there is shown in FIG. 1 a plasticfastener generally indicated at 10 integrally attached to a moldedplastic article 12. A variety of plastic materials may be employed thatpossess the requisite flexibility, strength, ease of molding and costbenefits; the presently preferred material is nylon. Fastener 10incluces an integral lug 14 that commences at a first end 16 thatintersects a first surface 18 of plastic article 12 substantially atright angles and extends outwardly therefrom, terminating in a distalend 20. Outer wall 22 extending between the ends of lug 14 preferablyhas a circular cross-section and a smooth surface for ease of insertion.The intersection of outer wall 22 and distal end 20 preferablyapproximates a right angle, but may be slightly rounded for ease inmolding.

A hollow core 24 is provided approximately at the center of lug 14 andextends from the distal end 20 of lug 14 towards plastic article 12.Core 24 terminates beyond first surface 18 and preferably in theinterior of plastic article 12 at end 26. Alternatively, core 24 mayextend completely through plastic article 12, but preferably end 26 isclosed to avoid problems with dust, etc. The shape of inner wall 28which defines core 24 is circular at least adjacent end 20. Wall 28preferably is slightly tapered inwardly from end 20 towards end 26,e.g., a few degrees. However, the diameter of outer wall 22 at end 20(D₁) is substantially the same as the diameter at end 16 (D₂) sinceouter wall 22 is not tapered. It can be seen that the length of lug 14(L₁) is less than the length of core 24 (L₂). As pointed out above, lug24 is flexible and is formed of a compressible material.

FIG. 3 illustrates one preferred embodiment of this invention whereinthe plastic article is a cover for a part of a seat belt retractor.Retractor 30 includes a frame 32 to which a spool 33 of seat beltwebbing is attached. As is conventional in such retractors, a rewindspring (not shown) is employed to bias the seat belt webbing towards theretractor. The rewind spring is typically located on the exterior offrame 32 and is contained within a spring cover 34. Cover 34 has agenerally circular shaped flat bottom wall 36 and a side wall 38 whichextends outwardly from bottom wall 36 and terminates in a circularmounting flange 40 which extends radially from side wall 38. Flange 40is provided with a plurality of integral fasteners 10 in the form asshown in FIG. 1. Retractor frame 32 is provided with corresponding roundapertures (aperture 42 in FIG. 4) for engagement by lugs 14. Thediameter of the apertures is chosen to be somewhat less than thediameter D₁ of lug 14 so that the lugs are held by an interference fitwithin the apertures and are held under compression.

As shown in FIG. 4, it can be seen that end 28 of lug 14 extends intoaperture 42 and terminates beyond inner wall 44 of frame 32; however,end 28 may alternatively be flush or inset with wall 44. The lower wall46 of flange 42 adjacent end 16 of lug 14 (which corresponds to surface18 in FIG. 1) forms a shoulder around lug 14 and is abutted againstouter wall 48 of frame 32.

An example of the dimensions of lug 14 are as follows: L₁ =3.8 mm, L₂=4.3 mm, D₁ =4.3 mm, D₂ =4.3 mm, diameter of core 24=2.8 mm, and thethickness of flange 40=1.2 mm. The degree of taper is 3°. A typicaldiameter of aperture 42 is 4.04 mm in a wall having a thickness of 3 mm.

Although in the above description the plastic article has been referredto as a cover for a retractor rewind spring, the cover may also be acover for a tension-relieving device, for a gear structure or otherplastic article.

The fastener of this invention exhibits many advantages over the priorart.

The fastener of this invention exhibits a higher extraction force thanprior art fasteners, such as on the order of about 50% higher. At thesame time, the insertion force is lower, such as on the order of about40% lower. Thus, articles incorporating the fasteners are easier toassemble. The high extraction force is retained over a considerablerange of interference fit, such as about 0.004 to 0.024 inches (0.1 to0.6 mm). As a result, the dimensions of the fastener, particularly theoutside diameter and wall thickness, do not require close tolerances,thereby reducing manufacturing costs. In addition, the extraction forceis not adversely affected by heat and humidity as is the prior artfastener; in fact, the extraction force of the present fastener isincreased under such conditions. Furthermore, the extraction force ofthe fastener is not substantially adversely effected by repeatedinsertions and extractions. For example, on the order of 90% of theextraction force is retained after two insertion-extraction cycles.Thus, the parts incorporating the present fastener can be reworked,increasing the salvage rate and further reducing manufacturing costs.

It is to be understood that variations and modifications of the presentinvention may be made without departing from the scope thereof. It isalso to be understood that the present invention is not to be limited bythe specific embodiments disclosed herein but only in accordance withthe appended claims when read in light of the foregoing specification.

We claim:
 1. In a molded plastic article adapted to be attached by anintegrally molded resilient plastic fastener to a cooperating parthaving an aperture for receiving said fastener, the improvementcomprising:said plastic article having a first surface, an oppositesecond surface and a solid central portion; and said fastener comprisingan integral lug extending from said first surface outwardly andterminating in a distal end, said lug in cross-section having a circularoutside surface of constant diameter along its length, the portion ofsaid first surface which intersects said lug defining a shoulder adaptedto surround said aperture, the central portion of said lug having anaxial hollow core, said core being surrounded by a solid wall portion,said hollow core extending from a first end adjacent said distal end toa second end which is located interiorly of the first surface so as atleast to extend into said solid central portion of said plastic article,the length of said core being greater than the length of said lug andthe width of said core adjacent said first end being greater than thewidth of said core adjacent said second end.